The present invention relates to an optical cell or an optical detection system which are suitable for the optical detection of samples optically with high sensitivity. The present invention is also concerned with an optical cell or an optical detection system which are suitable for optical detection of samples such as protein, sugar and nucleic acid which have been isolated by a separating means such as, for example, a liquid chromatography system or a capillary electrophoresis system.
Electrophoresis is effective as means for separating and analyzing biological substances such as DNA, protein and sugar. Heretofore, agalose or polyacrylamide gel have mainly been used as mediums for electrophoresis. For examples, these gels have been used in the form of a slab gel between two glass plates or a capillary gel in the interior of capillary. Also, studies have been being made about a method of using a buffer alone without using gel as a medium for electrophoresis. At present, a capillary electrophoresis using a capillary of not larger than 100 .mu.m in inside diameter is attracting attention of many concerns.
In electrophoresis using capillary such as, for example, capillary gel electrophoresis or capillary zone electrophoresis, since the capillary diameter is small, the surface area per volume of a migration medium is large and therefore the diffusion of joule heat is easy. Consequently, the rise of temperature is prevented even under the application of a high voltage and as a result it is possible to apply a higher voltage in comparison with the ordinary type of electrophoresis. That is, a high separating performance can be attained in a short time. Besides, because the capillary diameter is small, a small absolute amount of sample suffices, which is several ten nl (nanoliter) or so, and thus the capillary electrophoresis is suitable for microanalysis. For the detection of a sample isolated by electrophoresis there has been used an absorbance detected by a UV detector or a fluorescence intensity measured by a laser-excited fluorescence detection system. The capillary electrolysis is described, for example, in "Bunseki, No. 8" pp. 599-606 (1991) (published by the Japan Society for Analytical Chemistry).
Generally, in the electrophoresis using capillary, the concentration sensitivity [a detection limit (sensitivity) expressed in terms of a sample concentration] is insufficient, and fluorescence detection using laser beam as an excitation light has been studied as an attempt to attain a higher sensitivity. Studies have also been made about the method of measuring with a higher sensitivity an absorbance which is deficient in sensitivity. For example, FIG. 12 shows a conventional method which is described in "Analytical Chemistry, 63," pp. 1372-1376 (1991). In this conventional method, a silver reflection coating 102 and a protective coating 103 therefor are formed successively on an outer surface of a glass part 101 of capillary, and there are provided two very small windows for light incidence and exit, which are a window 105 of incident light and a window 106 of transmitted light and which are free of reflection coating. The windows 105 and 106 are formed in positions spaced from each other in the direction of capillary axis so that the incident light from the incident light window 105 may not directly go out straight from the transmitted light window 106. During measurement, a helium-neon laser beam 107 is made incident from the window 105 obliquely at a predetermined certain angle .theta. of incidence, then as shown in FIG. 12, it is reflected multi-reflectionwise in the capillary interior by the reflection coating and is allowed to pass a sample solution part 104 in a large number of times. Thus, the light path length is made longer to increase the amount of light absorbed. Thereafter, the output laser beam from the transmitted light window 106 is sensed to determine an absorbance.